An example of a switchable window is an electrically switchable window which changes light transmission properties in response to an electric field. Known uses of this technology include windows and/or glazings in vehicles, commercial buildings (e.g., offices, conference rooms, lobbies, buildings, store fronts, etc.), and/or residential buildings.
A transparent window, which is always fully transparent, may not be desirable in certain instances. For example, sunlight through a vehicle window may result in glare to the vehicle's passengers and/or excessive heat in the interior of the vehicle. Thus, a preferred window may allow some light to be transmitted at advantageous times and to allow light to be blocked at other advantageous times. To control light transmission through a window, some windows contain a photochromic or thermochromic material which changes its transmissive properties based on the amount of light incident on the material. These changes in transmissive properties are always automatic and cannot be overridden by human or other intervention.
Liquid crystals (LCs)—for example, polymer dispersed liquid crystals (PDLCs) and plasma addressed liquid crystals (PALCs)—vary the intensity of light transmitted through a liquid crystal medium/layer by changing the orientation of the liquid crystal molecules in the medium/layer in response to an electric field. A constant electric field may be applied by a direct current (DC) voltage. Alternatively, the polarity of the electric field may be periodically switched by application of an alternating current (AC) voltage.
The applied voltage may be electrically connected to a controller (e.g., electronic control unit, timer, switch, etc.) to be activated automatically with changing environmental or other conditions, or be activated via a switch by a human operator. LCs may even allow intermediate states between an “on” (transmissive or partially transmissive) state and an “off” (opaque or substantially opaque) state by varying the intensity of the electric field of the liquid crystal medium/layer.
PDLCs are typically made by inducing phase separation in an initially homogeneous mixture of liquid crystal and monomers. Preparation of PDLCs involves a phase separation, which can be triggered by polymerization of the monomer matrix by either ultraviolet (UV) or thermal curing, or even rapid evaporation of solvents. As the monomer polymerizes, the liquid crystal phase separates into microscopic droplets or domains or pockets surrounded by the walls of the cured polymer matrix, which provides a “backbone” to hold the LC. The mixture of cured polymer and LC are held together between two sheets of polyethylene (PET), often coated with transparent conducting oxides (TCOs) through which an electric field is applied. When unaddressed (e.g., when no voltage and/or voltage below a LC threshold voltage is applied), the nematic texture within the domains of the LC is randomly oriented with respect to other neighboring domains, and the display appears whitish and/or opaque caused by the scattering of light.
FIG. 1(a) illustrates a related art PDLC glass window 100 in an off state. Two glass substrates 102a, 102b are provided. A conductive coating 104 is applied to the respective inner surfaces of the outer substrates 102a and 102b. A plurality of liquid crystal (LC) droplets 108 are disposed within the polymer mixture 106. When no voltage is provided, the droplets 108 are randomly oriented, and incident light I refracts off them, causing the scattering of light in the directions shown by the dashed arrows.
In the addressed state (when voltage above the threshold voltage is applied to the liquid crystal layer), the nematic texture in different domains align with the electric field, thus allowing for a clear state as shown in FIG. 1(b). FIG. 1(b) is a related art PDLC glass window 100 in an “on” state. FIG. 1(b) is similar to FIG. 1(a), except that a voltage V is applied to the PDLC layer via conductor 104 and one or more bus bars (not shown). The voltage causes the liquid crystal droplets of the PDLC layer to align substantially parallel to the electric field, allowing incident light I to pass through the window 100 in providing for a clear state.
Although such techniques have represented an improvement in some windows, there still are certain drawbacks. For example, exposure to radiation from the sun may cause degradation of the PDLC. This degradation may be exacerbated with temperature increases in the PDLC.
Degradation of the PDLC may increase the response time of the PDLC which may contribute to a flicker noticeable to the human eye. The degradation may increase the scattering of light waves which may create contribute in part to a residual haze and cause a “browning” of the window when the PDLC is in the clear state.
U.S. Patent Document 2009/0115922 to Veerasamy, the entire contents of which are hereby incorporated herein by reference, attempts to overcome disadvantages through the use of a low-emissivity (low-E) coating.
FIG. 2 is a cross-sectional view of a related art window according to an embodiment of U.S. Patent Document 2009/0115922. In the window of FIG. 2, two substrates (e.g., glass substrates) 202, 204 are provided, including an outer substrate 202 and an inner substrate 204. A low-E coating 206 is deposited on the inner surface of the outer substrate 202. However, the transparent conductive oxide (TCO) layer 212 for applying voltage across the PDLC layer 214 is located between the PDLC layer 214 and the low-E coating 206.
First and second laminate layers 208 are provided. First and second polymer-based (e.g., PET) layers 210 are provided on the inner surfaces of the respective first and second laminate layers 208. A switchable PDLC layer 214 is sandwiched by first and second substantially transparent conductive oxide (e.g., TCO) layers 212. The TCO layers may be sputtered onto one or both surfaces of the PDLC 214 and/or the respective surfaces of the first and second polymer-based layers 210.
The low-E coating 206 of U.S. Patent Document 2009/0115922 reduces the long-term degradation of the LC layer 214. The additional layer 206, however, increases the thickness, manufacturing time, and manufacturing cost of the window.
Therefore, it will be appreciated that there is a need in the art for coated articles that overcome one or more of these and/or other disadvantages. It also will be appreciated that there is a need in the art for improved switchable coated articles for use in, for example, vehicle windows, insulating glass window units, etc.